CLONE_NEWNS and bind mounts to make "chroot" jail

[Leibowitz, Michael]

I have been trying to use the combination of CLONE_NEWNS and bind mounts
to create a better (than) chroot jail.  I wish to have the ability to
bind (ro will be possible in the future, I understand) certain
directories into the jail (perhaps /bin, /lib, /usr), but not have
parallel directories in the jail (no /etc, ..., /home). 

I have heard that this should be possible, but have yet to get a working
solution.

I have tried something analogous to:
chdir("/jail");
unshare(CLONE_NEWNS);
/* mount(8) syntax given for simplicity, but mount(2) used below */
mount --bind / /jail/old_root
mount --bind /jail/old_root/bin /jail/bin
mount --bind /jail/old_root/usr /jail/usr
mount --bind /jail/old_root/lib /jail/lib
mount --bind /jail /               # does nothing?
umount2("/old_root", MNT_DETACH);  # never happens.
exec("bin/sh");

When bin/sh runs, I can still see old_root from /jail and the bind of
/jail over / seems to have not done anything.  

Is it possible to create such a jail with bind mounts?  Is there a
recommended method for doing so?  Thank you for your time.

--
Michael Leibowitz
Software Engineer, UMG
Intel Corporation
michael.leibowitz at intel.com


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